In the field of radiation measurement devices, one of the important indicators of the device performance is an ability to measure photon energies of gamma rays, X-rays or the like with high accuracy. As one photon of coming incident generates an amount of electric charge or light emission roughly in proportion to the photon energy, a semiconductor detector or a scintillation detector measures the photon energy by measuring the generated amount, respectively.
In the measurement, the semiconductor detector generates an electric charge, which is outputted as an electrical signal as it is. On the other hand, the scintillator generates light emission, which is converted to an electric signal by a photomultiplier tube or the like then outputted as a signal. In general, as the output signals from these detectors are weak, the detector output is input to and amplified by the preamplifier provided with integral capability. Measuring the pulse height or the peak value of the pulse signal from the preamplifier accurately means measuring the photon energy accurately.
It should be noted that an apparatus is called an analog radiation spectrometer that processes a pulse waveform through analog circuitry for determining the pulse height and then measure the photon energy.
When there are photons continually coming incident to the analog radiation spectrometer, pulse signals are continually generated. When a time interval between the pulse signals become shorter, a pulse waveform will be interfered by the preceding pulse waveform and the measurement accuracy of the pulse height will be deteriorated.
For example, if a coming pulse signal overlaps a decaying pulse signal of the preceding pulse, the pulse height of the coming pulse signal is less than the true value. A pole-zero cancellation circuit is used as an analog circuit to prevent this.
Also, if the analog radiation spectrometer is not appropriately adjusted, a baseline shift may occur when pulses are continually generated. A baseline restoration circuit is used to prevent this baseline shift.
Further, if the pulse intervals are extremely short, the coming pulse is superimposed on the preceding pulse that are not decaying yet and a pile-up phenomenon will occur. A pile-up rejection circuit is used to prevent this.
Patent Literature 1 discloses a pile-up correction circuit, as an analog circuit for detecting the pile-up and performing correction of the pulse height.
Patent Literature 2 discloses a pile-up rejection circuit.
In addition, Non-Patent Literature 1 describes techniques related to the pole-zero cancellation circuit (p.673), the baseline restoration circuit (p.677), and the pile-up rejection circuit (p.722).
On the other hand, there is a digital pulse-height analysis technique as a completely different approach. This is a technique to achieve the equivalent function as the waveform shaping with the arithmetic processing of digital values, by sampling and digitalizing the output of the preamplifier at constant intervals shorter than the transition time of waveforms. Even non-linear processing being easily realized with digital operation, many problems due to a short pulse interval can be resolved.
Non-Patent Literature 1 describes a digital pulse-height analysis technique (p.736) that always performs sampling in a shorter time than the time of waveform changes, that is, at a high speed.
In addition, Patent Literature 3 discloses a technique for arithmetic processing of sampled signals as described above.